The present invention relates to a phase control circuit, and in particular to a phase shifter adjustment apparatus for a microwave ferrite antenna system.
In the prior art, radar antennas used ferrite phase shifters to change the phase of microwave energy entering the antenna in order to direct the propagation slightly off boresight, thereby determining the direction in which the antenna must be moved when tracking a target. Alternatively, the energy may be transmitted along boresight and the beam directed off boresight when receiving only (LOR, i.e., lobe-on-receive). Generally, phase shifters used for this purpose contain two control coils which generate respectively an axial magnetic control field and a transverse magnetic control field in a low-hysteresis ferrite material, and current must be applied continuously to the coils during operation. The phase shift to energy passing in the transmit direction is approximately independent of coil current, whereas the phase shift to energy passing in the receive direction is highly dependent on coil current. Ordinarily, each phase shifter is driven at a constant current level, but of either polarity, depending on the direction in which the beam is to be aimed at that time. Rapid phase-switching speeds are desirable because more time is left for radar pulsing.
In one radar, which led to the invention, the parameters of the ferrite and manufacturing processes precluded interchangeability of phase shifters unless coil-current adjustments could be made internal to the phase shifter itself. Diodes and resistors were used in parallel with each coil such that, for a constant input current, the current in each coil could be adjusted by bleeding unwanted current past the coil. Four diodes and four selected resistors total were required to adjust for both current polarities of both coils. The increased drive current requirement overtaxed the existing driver's power dissipation, requiring redesign, and the LR time constant internal to the phase shifter were so long that the coil currents could not decay fast enough to complete the phase switching in the time alloted. Also, the bleed currents varied with temperature due to the diode thermal characteristics. The solution adapted in this system was to purchase phase shifters in matched sets of four (4), and replace all four (the entire set) if one malfunctioned, i.e., interchangeability was abandoned.
The present invention provides a means for full adjustment, and therefore full interchangeability, of low-hysteresis type microwave ferrite phase shifters without the degrading effects described above and with the added benefit of temperature compensation, if desired.